Abstract : A large number of studies have focused on the reactivity of the hypothalamic–pituitaryadrenal (HPA) axis
and the consequences of glucocorticoids (GC) in mediating life-history trade-offs. Although short-term
increases in GCs are viewed as adaptive, mobilizing energy substrates allowing animals to deal with
impending threats (e.g. stimulating hepatic gluconeogenesis, stimulating lipolysis, mobilizing amino
acids), few studies have actually measured the exact time-course of substrate mobilisation in response
to acute stress in natural conditions. We evaluated the hormonal and metabolic components of the stress
response to acute stress in 32 free-living king penguins (Aptenodytes patagonicus). We monitored changes
in blood GCs (corticosterone, CORT), glucose, lactate, ketone bodies (b-hydroxybutyrate), non-esterified
fatty acids, and uric acid in response to a standardized capture-restraint protocol lasting for up to
90 min. Furthermore, we tested whether the vigilance status of the animal (alert or asleep) affected its
perception of the capture, thereby modulating the hormonal and metabolic stress responses. The time
course of energy mobilisation followed the characteristic pattern expected from laboratory and theoretical
models, with a rapid depletion of those energy stores linked to rapid adrenergic responses (i.e. glucose
and ketone bodies), followed by a mobilisation of energy stores associated with the sustained
longer-term GC response (i.e. fats and protein stores). HPA reactivity was generally slower than reported
in other birds, and there was high inter-individual variability. Sleeping birds had higher GC and glucose
responses to acute stress, suggesting a more rapid mobilization of energy stores. Our results highlight the
importance of considering HPA and metabolic responses to acute stress against species-specific life history
and ecological relevant backgrounds